The present invention relates generally to vehicle steering systems and, more particularly, to a method and system for implementing improved active damping of vehicle steering systems.
In vehicles equipped with electric power steering (EPS) systems, a steering assist is provided (for example) by an electric motor coupled to the steering column or shaft. In order to provide a stable and precise feel for such steering systems, active input-dependent damping is typically provided. In particular, active damping systems (such as those presently implemented in EPS systems) utilize a vehicle speed dependent damping value that is multiplied by motor speed, and then scaled by a handwheel torque dependent scale factor in order to reduce the active damping torque at high handwheel torques. One example of such a system is described in U.S. Pat. No. 6,647,329 (assigned to the assignee of the present application), and which provides an input dependent damping component. This approach allows for adequate damping in a free control situation while also maintaining high assist levels during an aggressive driving maneuver.
In vehicles that have poorly damped chassis dynamics, large amounts of active damping correction are typically required of the EPS system. Using large amounts of active damping correction may create a “sticky” feeling on center. It has been recognized that while a large amount of damping may be required to provide stability in a large steering maneuver, as the handwheel (and vehicle) settles out and the handwheel velocity reduces, a smaller damping value may be used. An earlier implementation of this concept was achieved, not simply by introducing a damping term directly, but by scaling the back electromotive force (BEMF) voltage compensation term as a function of handwheel velocity to achieve the same effect. However, this initial approach made it difficult to intuitively tune in the vehicle, resulting in appropriate adjustments being made on a dynamometer. In another implementation of active damping, a direct scaling of a given damping value as a function of handwheel velocity (but not vehicle speed) was utilized. In other words, although the damping itself was a function of vehicle speed, the handwheel velocity dependent scaling was not vehicle speed dependent.
However, there still remains a need for providing an active damping system that obviates the need for a trade-off between damping performance and a sticky on-center feel, not only for EPS systems, but for other systems such as steer-by-wire, for example.